Experimental study on falling film evaporation characteristics of R-134a outside of a vertical enhanced tube

The main objective of the present study is to investigate the heat transfer characteristics of vertical falling film evaporation of refrigerant R-134a outside of a single smooth and enhanced tubes. The experiment was carried out under the conditions that spray density in the range of 0.058–0.44 kg m−1 s−1, corresponding inlet Reynolds number from 1000 to 6800. The evaporation temperature ranges from 6 to 12 °C and heat flux is between 10 to 40 kW m−2. Variations of heat flux, spray density and saturation temperature were analyzed. The obtained results revealed that the saturation temperature has little influence on enhanced tube, but it can improve the performance of the smooth tube markedly. Moreover, it was found that as the heat flux increases, the heat transfer coefficient of the smooth tube increases, while that of the enhanced tube significantly reduces. Nevertheless, the heat transfer performance of the enhanced tube still outperforms that of the smooth tube up to 2∼12 times. Besides, it was also found that spray density is an important factor for vertical falling film evaporation. The increase of the spray density appropriately can improve the heat transfer coefficients in both smooth and enhanced tubes, but the supply that is too much or too little both has adverse effects. Accordingly, the spray density should be kept in a optimum range, especially the minimum must be reached in order to prevent liquid film break and the dry-out patch. The present study provides a basis to investigate the application of enhanced tube in the vertical falling film evaporator.